Error in R: no applicable method for 'predict' applied to an object of class "regsubsets"

Question

I was trying to run codes from textbook ISLR. It is about Best Subset Selection:

library(ISLR) #contain data Hitters
library(leaps) #use regsubsets
regfit.full=regsubsets(Salary~.,Hitters)
predict(regfit.full)

Then R showed error:

Error in UseMethod("predict") :no applicable method for 'predict' applied to an object of class "regsubsets" 

I am using Windows RStudio Version 0.99.893, R-3.2.4revised-win. I really have no idea why the "predict" cannot apply to "regsubsets".

This is the first time I ask a question here, please let me know what I can do to help solve this problem. Many thanks in advance!

--------------------update---------------------------------------

Thanks to Zheyuan Li, I figured it out. There is no predict method for regsubsets. We need to write one ourselves. Below is one from internet:

predict.regsubsets = function(object, newdata, id, ...) {
    form = as.formula(object$call[[2]])
    mat = model.matrix(form, newdata)
    coefi = coef(object, id = id)
    mat[, names(coefi)] %*% coefi
}

other info:

> sessionInfo()
R version 3.2.4 Revised (2016-03-16 r70336)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows >= 8 x64 (build 9200)

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252    LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C                           LC_TIME=English_United States.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] ISLR_1.0  leaps_2.9

loaded via a namespace (and not attached):
[1] tools_3.2.4

> methods(predict)
     [1] predict.ar*                predict.Arima*             predict.arima0*            predict.glm               
     [5] predict.HoltWinters*       predict.lm                 predict.loess*             predict.mlm*              
     [9] predict.nls*               predict.poly*              predict.ppr*               predict.prcomp*           
    [13] predict.princomp*          predict.smooth.spline*     predict.smooth.spline.fit* predict.StructTS*         
    see '?methods' for accessing help and source code

> regfit.full
Subset selection object
Call: regsubsets.formula(Salary ~ ., Hitters)
19 Variables  (and intercept)
           Forced in Forced out
AtBat          FALSE      FALSE
Hits           FALSE      FALSE
HmRun          FALSE      FALSE
Runs           FALSE      FALSE
RBI            FALSE      FALSE
Walks          FALSE      FALSE
Years          FALSE      FALSE
CAtBat         FALSE      FALSE
CHits          FALSE      FALSE
CHmRun         FALSE      FALSE
CRuns          FALSE      FALSE
CRBI           FALSE      FALSE
CWalks         FALSE      FALSE
LeagueN        FALSE      FALSE
DivisionW      FALSE      FALSE
PutOuts        FALSE      FALSE
Assists        FALSE      FALSE
Errors         FALSE      FALSE
NewLeagueN     FALSE      FALSE
1 subsets of each size up to 8
Selection Algorithm: exhaustive